It is well known that multiple forward biased diodes can be stacked in series as electrostatic discharge (ESD) protection circuit in view of the low parasitic capacitance of the series connection of diode capacitances.
In BiCMOS technology this series connection of diodes can also be implemented by means of bipolar transistors, which also leads to a low leakage current. However, with a number of N bipolar transistors in series, whereby the base-emitter junction of each bipolar transistor is used as diode, the clamping voltage over the diode chain is increased to N times the built-in junction voltage Vbe(on) of a base-emitter junction. For advanced technologies (such as 130 nm SiGe BiCMOS) this clamping voltage N×Vbe(on) is too high because of the low collector-emitter breakdown voltage of the bipolar transistors and the possibly low oxide breakdown voltage of the gate dielectric of the MOSFETS in the circuitry to be protected by this diode-chain.